May 4, 2018 | By David
Researchers at the UK’s Diamond Light Source research facility, which is located at the Harwell Science and Innovation Campus in Oxfordshire, have been working on a new project to advance development of laser-based 3D printing techniques. In collaboration with scientists at I12, the Joint Engineering Environment Processing (JEEP) beamline and the Central Laser Facility, they have built a laser additive manufacturing (LAM) machine that operates on a beamline, allowing users to see into the heart of the process and revealing the various underlying physical phenomena that occur during LAM.
LAM processes are useful for creating 3D objects with complex geometries in a relatively short amount of time, and they work by directing a laser to selectively melt a bed of metal or ceramic powder, which then re-solidifies and fuses together in order to build up the desired shape, layer-by-layer.
The cooling rates for the LAM process are extremely rapid, and since they are unlike conventional manufacturing processes it is difficult to know what the optimal conditions are to obtain the best possible properties. This lack of knowledge is delaying the uptake of LAM in the production of safety-critical engineering structures like turbine blades, energy storage systems and biomedical devices. We’ve reported before on research projects being carried out in order to improve the metal 3D printing process, with more precise knowledge about what goes on in the melt pool being generally the most promising area of study. This latest study was one of the most advanced so far, as it took advantage of the Diamond Light Source facility’s powerful synchrotron imaging system.
According to Professor Peter Lee from The University of Manchester, who is leading this project, "The LAM process is very fast, taking place in milliseconds, and to investigate we need microsecond resolution, which can only be achieved with the brilliance of a synchrotron. It allows us to follow the process from powder, through melting and then solidification back into the final solid shape. On JEEP we are investigating the superalloys used in aeroengines, and we need the high energy, hard X-rays produced there to see inside them."
The team developed a novel LAM process replicator, known as the LAMPR. This allowed them to image and quantify the formation of the melt track, as the layers were printed during the AM process. The LAMPR was designed to fit on the beamline and it mimics the way a commercial LAM system works, with additional windows that are transparent to X-rays to allow scientists to see right into the heart of the LAM process as it takes place. The LAMPR shed light on various important mechanisms in the LAM process, including the formation and evolution of melt tracks, spatter patterns, the denuded zone (a powder-free zone) and porosity in the deposited layers.
(images: Diamond, Nature Communications)
A key finding from the LAMPR was that surface porosity in LAM 3D printed objects is often caused by a pore-bursting mechanism, where pores near the surface escape into the atmosphere, leaving behind a surface depression. This disproved a previous hypothesis that suggested incomplete melting was to blame. The team also found that pre-melting caused by surface tension led to release of metal vapours and heating of inert gas, which was another potential source of defects, forming a plume which ejects powder and molten droplets away from the main track.
The team used their research to create a process map, which illustrates how to tune and optimize the various parameters of the LAM process to produce a better quality product with minimal trial and error. The results of their research were detailed in a paper entitled "In situ X-ray imaging of defect and molten pool dynamics in laser additive manufacturing", published in the journal Nature Communications.
Posted in 3D Printing Technology
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